Tone receivers are in use throughout the communications industry to fulfill a variety of signalling needs. Typically, they employ resonant electrical or electromechanical circuits to provide selectivity or multi-channel separation which must be critically adjusted during manufacture and carefully monitored in service to assure dependable operation. There are two basic categories of tone receivers. The first category is used for data reception. The second category includes the types typically used for supervisory control.
The present invention is of the second type and is intended for use on ordinary telephone subscriber lines to fulfill a need for supervisory control on the subscriber premises. While the present invention has application in numerous environments and for many purposes, it is specifically designed for use on multi-party telephone subscriber lines to allow selective activation of automatic residential utility meter reading transponders located at each subscriber residence for use in conjunction with an automatic meter reading transponder which is disclosed in copending U.S. Pat. applications Ser. No. 565,974, filed Apr. 7, 1975, entitled "Transponder for Transmitting Data from Digital Encoding Transducers over a Telephone Line".
The present tone receiver is provided to respond to only one of a plurality of tone frequencies by utilizing a digital method of responding to the one frequency. This is analogous to a very high Q tuned (L/C) circuit, but without the deficiencies normally exhibited by such circuits. For example, the channel bandwidth is essentially independent of the input signal amplitude. Since, in the present device, there are no resonant circuits in the receiver, the receiver is inherently stable.
In addition, typical multi-party telephone lines may include "bridge lifters" in series with each subscriber pair so that loading of an active line by idle lines is minimized. Bridge lifters are essentially saturable reactors placed in series with each subscriber pair which exhibit very high impedance to voice frequencies. Ordinarily, off-hook supervisory DC currents effectively saturate the cores of the bridge lifters sufficiently to drastically reduce their impedance to a level which does not impair voice communication over an active pair. Where outgoing (from central office to subscriber) voice band signals are involved, however, they severely attenuate the signals since there is no DC current flow in an on-hook line. Common practice, where signalling over such lines is required, is to use abnormally high level signals at the lower frequencies to overcome the attenuation. While this is an effective method, it has the disadvantage of increasing crosstalk to other pairs in the cable. Bandwidth, sensitivity, and the relationship between these two characteristics are important considerations in the design of a device to be connected to a subscriber telephone line. This is particularly important where the device is to be located at the subscriber end of the loop.
If the subscriber line is equipped with MF (multifrequency) type service, the device will be subjected to relatively high level audio signals and must not be falsely activated by signals originated at the station end. On the other hand, the device must respond to low level signals if it is to operate on the longest loops. False activating cannot be tolerated due to the interference it might cause. Similarly, the relatively high level signals it might receive on a short loop must not cause false activation. For those and other considerations, the present invention provides a device suitable for use in multi-party line environments.